The present invention relates to ceramic monolithic catalyst or catalyst supports, and, in particular, a process of manufacturing alumina-silica honeycomb catalysts or catalyst supports.
The petrochemical industries currently use a variety of pellet type structures formed of gamma alumina or other oxides, e.g., pellets, pills, beads, rings, trilobes, stars, and so forth, as catalysts or catalyst support media for catalytic reactions. These structures are typically formed by extrusion from batch mixtures of alumina or other selected oxides, followed by drying and calcining. Pellet beds, however, tend to exhibit relatively high flow resistance, and also develop preferential flow paths which exhaust portions of the catalyst while leaving other portions relatively unused.
Monolithic catalyst supports, such as honeycomb structures, have been proposed as an alternative to the aforementioned pellet type structures for providing several advantages. These including lower pressure drop, greater surface area, higher yield, higher selectivity, and more compact reactor designs. Generally, ceramic honeycombs are used in applications in which the monolithic substrate serves simply as a physical structural support for a chemically active, high-surface-area catalyst coating (e.g., a washcoating of alumina deposited on the channel walls of the ceramic honeycomb.) However, for many applications porous washcoatings are inadequate and catalysts or catalyst supports made up mostly or entirely of active, high surface area material must instead be used.
One such application is the hydro-desulfurization of fossil fuels in the petrochemical industry to make low sulfur gasoline and diesel fuels. Since the reaction kinetics are the slow step in such processes, it is important to provide a relatively large accessible BET catalyst support surface (more catalyst sites in a given volume) in order to allow the most effective use of reactor volume. This in turn requires that the entire volume of the catalyst or catalyst support structure be made of active, high-surface-area material, and that the pore structure of the material be such that that the reactants can diffuse in and products diffuse out of the volume of the catalyst support effectively over relatively long distances.
U.S. Pat. No. 4,631,267 relates to the manufacture of extruded honeycombs of alumina, silica and titania compositions that incorporate precursors for permanent silica, alumina and titania binders in powdered alumina, titania or silica extrusion batches. The precursors for the permanent binders are generally liquid solutions or dispersions of oxide-yielding compounds such as titanium isopropoxide or silicone solutions or hydrated alumina slurries, these being converted to small crystallite bonding deposits of the respective oxides on firing.
U.S. Pat. No. 6,365,259 relates to the manufacture of extruded honeycombs of gamma alumina compositions absent permanent binders in the starting batches. The forming process includes providing a powder component of alumina powders including at least one anhydrous alumina powder, which is combined with water, an acid, and a plasticizing temporary binder to form a plasticized mixture. The plasticized mixture is extruded into a honeycomb structure, which is dried and fired.
However, there still remains a need for a process of making silica-alumina catalyst supports of high-surface area, in an effective and economic manner.